Seamless tubes are customarily manufactured by the hot rolling of steel ingots which have been heated to about 1200.degree.-1250 C. The main shaping of the rolling process takes place just below the ingot drawing temperature and therefore at very high temperatures. The recrystallization of the structure caused by the shaping leads, because of these high temperatures, to strong grain growth and, accordingly, to a corresponding impairment of the toughness of the material. Therefore, up to now it has always been considered necessary to follow the rolling process by a separate heat treatment in the form of a normalization or age-hardening process thereby obtaining a finer structure and improving the toughness properties by re-granulation.
Such a heat treatment requires a considerable expenditure of time and money so that it appeared desirable to avoid this additional treatment. In principle, it is possible by a targeted cooling, after the hot rolling, to produce a bainitic structure in the tube and, therefore, to substantially avoid both ferrite formation and martensite formation. A bainitic structure, in addition to providing high strength values, exhibits good toughness properties. This method has, however, scarcely been possible up to now on a large industrial scale since the control of the temperature during the accelerated cooling step could not be effected in such a manner as to assure the reaching of the bainite region.
This is due to the fact that the individual steel tubes invariably have temperatures which not only differ from each other at the end of the last rolling step but also show considerable differences in temperature over the length of the tube and even over the circumference of the tube. These differences in temperature amount typically to up to about 100.degree. C. and can be measured at the end of the accelerated cooling step in approximately unchanged amount. Therefore, in actual practice, a fixed cooling temperature cannot be achieved.
This, however, means that, due to the cooling, a bainite region is obtained in the desired manner only in individual zones of the tube, while in the other zones ferrite is produced (by excessively slow or insufficiently deep cooling) or else martensite (due to excessively deep cooling). As a whole, such tubes have toughness and strength properties which differ greatly from each other locally and they are thus not suitable for the intended use.
From Federal Republic of Germany 33 11 629 C2 a method is known, however, which permits the manufacture of high-strength seamless oil-field tubes of good toughness properties from a steel having the following composition:
______________________________________ 0.02-0.12% C 1.30-2.20% Mn max. 0.30% Mo max. 0.50% Ni 0.01-0.04% Ti 0.02-0.06% Nb 0.003-0.008% N 0.03-0.05% Al 0.001-0.003% B 0.001-0.010% S ______________________________________
the balance being iron and ordinary impurities.
In order to obtain a finely granular structure, the prior art method provides for a controlled final rolling at low temperatures (700.degree.-850.degree. C.), whereby a completely uniform starting temperature is assured by a previously effected equalizing annealing over the length of the tube after it has been prerolled in the hot.
After the final rolling, the tube is cooled with water and/or air to room temperature and finally also tempered in order to improve the strength properties. The intermediate or subsequent heat treatment means an additional expense with respect to technical plant and energy consumption.